There are three basic types of power transformers: an open ventilated dry or “open wound” type, a vacuum cast or vacuum encapsulated “solid cast” type, and an “oil filled” type. “Open wound” transformers are the oldest products and are typically made of either layer windings or, step/precision windings in combs. The completed coil or sometimes the entire unit is then dipped into varnish and a coating of environmental and mechanical protection material added per a “dip-and-bake” cycle. These units are the most polluting as they typically contain two petroleum based components (film insulated magnet wire and polyester winding combs) and utilize a third component (varnish) during their respective manufacturing processes. Further, the film insulated magnet wire is manufactured by applying a solvent based material to round or shaped conductor. The solvents or volatile organic compounds are volatilized or emitted into the atmosphere as the polymers are cured. The polyester winding combs are also petroleum based and are typically machined in-house, thus creating dust or air pollution. The final dip-and-bake process also emits about 50% of the solvents (VOC's or HAPs) into the atmosphere as the units are cured.
Vacuum cast or “solid” cast transformers are superior to open wound transformers. The coils are typically made of copper without using any film insulated magnet wire and do not require winding combs. The final encapsulation may include epoxy material and thus emits no VOC's. These transformers are made by casting and curing a thermosetting resin composition around conductor windings to form a resin body covering a coil. The resin body contributes dielectric properties as well as structural reinforcement to the conductor windings. The resin provides impact protection and helps dissipate heat from the coil uniformly.
“Oil filled” transformers typically utilize lower temperature paper based products as ducting to provide physical space between conductors and as a conduit for the insulating fluid. The ducting material is typically adhered to pressboard or a similar material using a secondary adhesive. The dielectric is achieved by flooding the entire unit in an electrically insulating fluid, petroleum or non petroleum based oil.